1. Field of the Invention
This invention relates to fuel burning apparatus and more particularly to a burner for firing a concentrated coal-water slurry, or fuel oil, in a utility or industrial boiler.
The drive for energy independence and, more particularly, the drive away from dependence upon foreign oil fuel sources, has led to the reevaluation of coal as an economical, near term oil-fuel substitute suitable for use in utility or industrial boilers designed to burn oil. Although fuel development research has been heavily focused upon coal-oil mixtures, coal-water slurries are economically more attractive fuels. Coal-water slurries, for example, are reportedly twenty to fifty percent cheaper than No. 6 fuel oil while, in comparison, the cost differential between coal-oil mixtures and No. 6 fuel oil is approximately five percent (Chemical Engineering, June 27, 1983, p.16).
Concentrated coal-water slurries are composed of a concentrated suspension, sixty-five to eighty percent by weight on a moisture-free basis, of pulverized coal or other carbonaceous fuels, in water containing a small percentage of chemical additives to stabilize the suspension. Such concentrated slurries differ from coal-water slurries containing smaller percentages of coal in that the concentrated slurries can be burned directly, i.e., without dewatering. The advantages of coal-water slurries as a boiler fuel include ease of handling, tranporting and storing a liquid-like fuel, low cost per unit of energy, independence of foreign oil, the capacity for integration with coal-water slurry pipeline systems, and technical and economic feasibility of retrofitting oil-designed boilers to burn coal-water slurries. In fact, the principal market for coal-water slurries, in the near-term, will be as a fuel for existing boilers originally designed to burn oil. The retrofitting of oil-fired boilers to use concentrated coal-water slurries necessitates the development of burners which can reliably and efficiently fire a slurry of pulverized coal and water or fuel oil.
Since a coal-water slurry behaves like a liquid fuel, it can be burned like fuel oil. Thus, the coal-water slurry is atomized and mixed with combustion air to form a stable flame front at or near the burner. It is most desirable to induce a high swirl at the burner as a means to stabilize the flame. The high swirl creates a strong recirculation of hot combustion products back into the ignition zone where drying and ignition of the coal particles contained in the incoming coal-water slurry droplets take place.
2. Description of the Prior Art
Previous suggestions have been made, for example, see U.S. Pat. No. 3,124,086, for burning a coal-water slurry which is not highly concentrated. As disclosed in U.S. Pat. No. 3,124,086, combustion air must be supplied at relatively high pressure which results in undesirable high pressure losses across the burner. In addition, the combustion air must generally be heated to a relatively high temperature.